1. Field of the Invention
Exemplary embodiments of the present invention relate to an apparatus and method for cancelling a feedback signal of a radio signal; and, more particularly, to an apparatus and method for cancelling a feedback signal of a radio signal in an on-channel repeater.
2. Description of Related Art
In general, a repeater is an electronic device that receives a signal from a transmitter and retransmits it at higher power. The repeater is generally installed at a fringe area where a signal of a transmitter becomes weak. Such a repeater solves a fringe area problem and expands a transmission area of a transmitter.
In a mobile communication field, a repeater is an auxiliary device that amplifies a weak signal between a base station and user equipment to provide a high speech quality to a user. A repeater was developed as a replacement of a base station and improves a speech quality at a shadow region where a signal of a base station cannot reach. Particularly, a repeater allows a service provider to minimize an investment cost because a repeater requires inexpensive installation and maintenance cost and occupies a small area.
In a mobile communication system, a repeater was introduced as a technology for overcoming a fringe area problem. Lately, a repeater has been widely used. Although a repeater was developed to have a limited and simple function of amplifying a signal, a repeater has been advanced to have multiple intelligent functions. That is, a repeater becomes a core technology for expanding a service coverage area and improving a data processing rate as well as reducing costs for installing base stations and maintaining a backbone communication network. As the advancement of a repeater technology, a new wireless communication system is required to support a typical repeater used in an existing wireless communication system.
FIG. 1 is a diagram illustrating a service provided using typical repeaters. In FIG. 1, a plurality of repeaters 102-105 use different frequencies to relay signals from a main transmitter 101.
As shown in FIG. 1, a main transmitter 101 transmits a signal with a frequency A. Each repeater 102 to 105 relays the signal from the main transmitter 101 using frequencies B, C, D, and E different from the transmission frequency A of the main transmitter 101. Since the repeaters 102 to 105 use different frequencies B, C, D, and E, it is necessary to allocate a plurality of frequency bands. Such a relaying method is ineffective in a view of frequency usage.
FIG. 2 is a diagram illustrating a service using on-channel repeaters. In FIG. 2, a plurality of repeaters use the same frequency.
As shown in FIG. 2, a main transmitter 201 transmits a signal through a frequency A. A plurality of on-channel repeaters 202 to 205 relay a signal using a frequency A which is identical to the transmission frequency A of the main transmitter 201. Such on-channel repeaters 202 to 205 cancel noises of a Radio Frequency (RF) signal transmitted from the main transmitter 201 by converting the RF signal to a baseband signal. After cancelling noise from the RF signal, the on-channel repeaters 202 to 205 modulates the noise cancelled signal to a RF signal and relay the RF signal using the same channel.
In order to stably provide such a service, a feedback signal cancellation apparatus of an on-channel repeater must be capable of suppressing a feedback signal below a predetermined level that guarantees a stable reception performance of a receiver. The feedback signal is generated due to a feedback channel formed between a transmission antenna and a reception antenna of a repeater. Also, the feedback signal cancellation apparatus must minimize a spectrum distortion of a restored signal in order to assure the stable operation of an on-channel repeater and a receiver.
Hereinafter, drawbacks of a typical feedback signal cancellation apparatus of an on-channel repeater will be described with reference to FIG. 3.
FIG. 3 is a diagram illustrating a typical feedback signal cancellation apparatus of an on-channel repeater.
Referring to FIG. 3, a typical feedback signal cancellation apparatus includes a reception antenna 301, a first Radio Frequency (RF) receiver 302, a subtractor 303, a replica generator 304, a second RF receiver 305, a RF transmitter 306, and a transmission antenna 307. The reception antenna 301 receives a RF signal transmitted from a main transmitter (not shown). The first RF receiver 302 down-converts the received RF signal to a predetermined band. The subtractor 303 receives the down-converted signal from the first RF receiver 302 and cancels a feedback signal from the down-converted signal by subtracting a replica of a feedback signal from the down-converted signal.
The RF transmitter 306 receives the feedback cancelled signal from the subtractor 303 and converts the feedback cancelled signal to a RF transmission signal as a repeater output signal. The RF transmitter 306 transmits the RF transmission signal through the transmission antenna 307.
The second RF receiver 305 receives the RF transmission signal from the RF transmitter 306 and down-converts the received RF transmission signal to a predetermined frequency band. That is, the second RF receiver 305 down-converts the RF transmission signal received from the RF transmitter 306 to a signal having the same frequency band as that of the down-converted signal outputted from the first RF receiver 302. Then, the second RF receiver 305 outputs the down-converted signal to the replica generator 304. The replica generator 304 generates a replica of the feedback signal based on the feedback cancelled signal from the subtractor 303 and the repeater output signal from the second RF receiver 305. The replica generator 304 feeds back the generated replica to the subtractor 303 to cancel the feedback signal.
FIG. 4 is a detailed diagram illustrating a feedback cancellation apparatus of an on-channel repeater of FIG. 3.
In FIG. 4, a reception antenna 401, a first RF receiver 402, a subtractor 403, a second RF receiver 407, a RF transmitter 408, and a transmission antenna 409 correspond to the reception antenna 301, the first RF receiver 302, the subtractor 303, the second RF receiver 305, the RF transmitter 306, and the transmission antenna 307 of FIG. 3. Accordingly, the detailed descriptions thereof are omitted herein.
Referring to FIG. 4, the replica generator 406 includes an adaptive filter 404 and a filter coefficient generator 405.
The filter coefficient generator 405 generates a filter coefficient based on a feedback cancelled signal from the subtractor 403 and a repeater output signal from the RF receiver 407.
The adaptive filter 404 receives the filter coefficient from the filter coefficient generator 405 and the repeater output signal from the second RF receiver 407, and generates a replica of a feedback signal using the filter coefficient and the repeater output signal. After generating the replica, the adaptive filter 404 feeds the replica back to the subtractor 403.
The filter coefficient generator 405 uses a Least Mean Square (LMS) adaptive algorithm to calculate the filter coefficient h(n). Particularly, the filter coefficient generator 405 uses Equation 1 below.h(n)=h(n−1)+μ·e*(n)·y(n)h(n)=[h0(n)h1(n) . . . hM-1(n)]T h(n−1)=[h0(n−1)h1(n−1) . . . hM-1(n−1)]T y(n)=[y(n)y(n−1) . . . y(n−M+1)]T  Eq. 1where,
e(n) represents the feedback cancelled signal from the subtractor 403,
h(n−1) represents a previous filter coefficient,
y(n) is the repeater output signal vector which is received at the second RF receiver 407,
μ represents a constant deciding a convergence speed,
M represents a length of a filter,
* denotes a complex conjugate, and
T denotes a transpose.
The adaptive filter 404 calculates a replica r(n) of a feedback signal by filtering the repeater output signal vector y(n), which is received at the second RF receiver 407 based on the filter coefficient h(n) generated by the filter coefficient generator 405. Particularly, the adaptive filter 404 uses Equation 2 below.r(n)=hnT·y(n)  Eq. 2
The subtractor 403 cancels the feedback signal by subtracting the replica r(n) from the output signal x(n) of the first RF receiver 402 using Equation 3 below.e(n)=x(n)−r(n)  Eq. 3
As shown, the typical feedback signal cancellation apparatus of FIG. 3 suppresses the feedback signal below a predetermined level that assures a stable performance of an inner equalizer. However, the typical feedback signal cancellation apparatus of FIG. 3 causes distortion of a restored signal due to the influence of a DC pilot of a vestigial side band (VSB) signal.
Such a spectrum distortion problem of the typical on-channel repeater may deteriorate the overall performance of an on-channel repeater and a receiver.
Accordingly, in order to stably provide a service through an on-channel repeater, it is required to develop a technology for minimizing the spectrum distortion of the restored signal of the feedback signal cancellation apparatus and suppressing feedback signals below a predetermined level that assures a stable performance of a receiver at the same time.